A tactic for trapping intermediates in the nitrogenase reaction

Hydrogenation of N₂ to NH₃ by the enzyme nitrogenase necessarily involves intermediates with H-N bonds. I propose a tactic for trapping these intermediates, through formation of stabilising hydrogen bonds using contiguous hydrogen bond acceptors in the reaction space. Wild type protein contains an aprotic reaction space, significantly bounded by α-70^Val. The carboxylate sidechain of aspartate substituted at this position is capable of forming very good O⋯H-N hydrogen bonds with intermediates in proposed mechanisms. Alternatively, serine at this position can form O-H⋯N hydrogen bonds with N lone pairs on some intermediates. These expectations are elaborated with density functional optimisations of nine proposed intermediates and one transition state in the substituted proteins 70^Asp, 70^Ser and 70^Glu. Possible outcomes are discussed, with the expectation that experiments in which these three substituted proteins undergo turnover with N₂ could yield trapped intermediates on a time scale that permits their structural characterisation by crystal diffraction or cryoEM.